Process for treating paperboard



United States Patent M US. Cl. 117-60 6 Claims ABSTRACT OF THEDISCLOSURE A method for treating paperboard so as to produce a boardwhich is substantially impervious to the penetration of molten waxapplied thereto, the applied wax remaining on the surface of the boardand thereby preventing water, used in solidifying the wax, from wettingand softening the thus-treated and waxed coated paperboard whichinvolves first applying to the board an aqueous solution of borax, andthen applying an aqueous solution of polyvinyl alcohol, followed by thesteps of applying a coating of hot wax to the thus-treated paperboardand setting the Wax by chilling it with cold water.

This invention relates to the production of an improved Waxed cartonwhich is particularly suitable for packaging frozen foods and the like.

In the production of wax-coated paperboard which is to be used forcartons and the like, it is customary to size the board with starch orsimilar materials, prior to wax application, in order to keep the moltenWax from penetrating into the paperboard. The Wax application isconventionally followed by the use of cooling water for setting the hotwax. If the molten wax has penetrated into the paperboard, and notformed a continuous film, then the cooling Water itself penetrates dueto the vacuum created by the reduced temperature, thus Wetting,.andthereby softening the board, which is undesirable.

But the use of starch sizing alone does not satisfactorily preventsubsequently applied hot wax from penetrating through the said coatingor sizing into the board. And, while it has been suggested in the art tocoat paper with polyvinyl alcohol, or mixtures of polyvinyl alcohol,with coagulating agents including borates, in order to provide greaseresistance, this technique likewise is not entirely satisfactory toeffectively keep molten wax which is applied over such sizing or coatingfrom penetrating into the paperboard.

It is a principal object of this invention to provide a method fortreating paperboard so as to produce a board which is substantiallyimpervious to the penetration of molten wax applied thereto, the appliedWax remaining on the surface of the board and thereby preventing water,used in solidifying the wax, from wetting and softening the thus treatedand Wax-coated paperboard.

In the practice of our invention, as illustrated in the followingdescription of an embodiment thereof, We utilize paperboard which hasbeen previously starch treated in conventional manner at the size pressand has a top surface which has been clay-coated in accordance with conventional procedures so as to provide a printing surface. Such a boardis first contacted with an aqueous solution of borax and then with anaqueous solution of polyvinyl alcohol. The polyvinyl alcohol-boraxcoating can be applied to either or to both sides, but in a preferredembodiment, is limited to the back, as contrasted to the claycoatedside. When solutions of borax and polyvinyl alcohol come into contact,it appears that a reaction occurs which results in crosslinking borateions and polyvinyl alcohol into a complex polymer, forming a rigidthree-dimen- 3,485,656 Patented Dec. 23, 1969 sional gel. For thisreaction to occur on paperboard surfaces to an extent or degree mostsuitable for the present invention, there should be provided aconcentration of from about 0.15 to about 0.25 pound of borax (10 11101)per thousand square feet of paperboard, and a sufficient concentrationof polyvinyl alcohol, preferably of the fully hydrolyzed type, to givecontinuity to the gel structure, that is to say, enough so that thereaction product fills the pores rather than coats the Walls. Borax isavailable as either 5-mol or 10-mol, designating whether 5 or 10 mols ofWater of crystallization are combined with sodium tetraborate. On achemically equivalent base (78.4 pounds of 5-mol:100 pounds of 10-mol).Either grade of borax may be used.

Polyvinyl alcohol is available in a range of polymers of varyingsolution viscosities and degree of hydrolysis, and is provided byhydrolyzing polyvinyl acetate polymers, the percentage of availableacetate groups replaced by hydroxyl groups describing the degree ofhydrolysis.

In general, the polyvinyl alcohol of highest viscosity and greatestdegree of hydrolysis will be most efficient for the surface treatment ofpaperboard in accordance with my invention. High viscosity slows downpolyvinyl alcohol penetration and a fully hydrolyzed polyvinyl alcoholreacts more extensively than a partially hydrolyzed polyvinyl alcohol.We have successfully used, as the polyvinyl component, Vinol No. 165,manufactured by the Air Reduction Company.

The F. viscosity of the polyvinyl alcohol may range from 2 up to 20,000cps. depending on the grade and concentration of polyvinyl alcohol, butwe prefer to utilize polyvinyl alcohol having a 95 F. viscosity ofbetween about 20 and 200 cps. The viscosity can be kept at the desiredlevel by controlling the pH of the solution, by use of phosphoric acid,for example, so that it is about 7 or below.

Excellent results are obtained using an aqueous solution of TechnicalGranular Borax ranging in concentration from about 0.5 to 20%,preferably 25-30%, and applying it to the surface of the paperboard byany conventional method, at the calenders. The treatment at thecalenders is immediately followed by an application of an aqueoussolution of polyvinyl alcohol, suitably by use of a trailing blade. Theconcentration of the polyvinyl alcohol may vary over a wide range, forinstance, from about 0.5% to about 20%, depending on the amount ofholdout desired; I have obtained very good results using a 2-3% aqueoussolution. As an illustration of the effectiveness and efficiency of thisinvention, I have obtained a very satisfactory degree of Wax holdoutplus grease resistance, using 0.06-0.20 pound of polyvinyl alcohol, and0.15 to 0.25 pound of borax per thousand square feet of paperboard. Theamount of polyvinyl alcohol on the board may vary from about 0.06 to1.50 pounds per thousand square feet; and the borax may vary irom about0.10 to about 2.0 pounds per thousand square eet.

While variations in application of the borax and of the polyvinylalcohol solution will of course be apparent to those skilled in the art,we find that the borax solution is applied most satisfactorily by meansof a calendar roll. Attempts to apply borax at the size press we havefound result in the destruction of internal sizing because of excessivepenetration of the borax solution. With internal sizing destroyed thepaperboard is difiicult to dry and excessive amounts of subsequentlyapplied calender solutions soak into the sheet. In accordance with thisinvention the borax solution is applied to paperboard which has beenpreviously coated with starch, and/ or clay. The precoat (starch or clayover starch) cuts down penetration of the borax solution. The surfacefinish at the calender is also better than at the size press to furtherminimize borax solution penetration. The polyvinyl alcohol solution isapplied to the board surface immediately after the calender applicationof the borax solution and while the borax solution is still wet, excesssolution being removed preferably by means of a trailing blade, or by anair doctor or blade. Because the polyvinyl alcohol is applied while theborax solution is still wet, the polyvinyl alcohol mixes with the boraxto cause gelling to take place on the paperboard surface and before itis permitted to migrate. There is thus produced a gel network on theboard surface which bridges all large surface pores and provides ahighly desirable level coating.

The thus treated paperboard may then be printed on its clay coated sideand/or cut and passed through any conventional waxing equipment whichrequires either water baths or water curtains to chill and set the wax.

The outstanding characteristic of our coating lies in its ability toreceive and maintain the Wax on the surface in a continuous film,thereby preventing negative pressures which are developed throughexpansion of water and gases by the heat of application (of the wax),from forcing water into the sheet resulting in a limp and inferiorcarton.

In the aforesaid description of our invention it will be understood thatby wax we mean to include all waxes both natural, petroleum andsynthetic waxes as are conventionally used in coating paperboard andwhich are susceptible to the above described coating procedures.

Other variations of our invention may include, for example, theutilization of paperboard which has been conventionally clay-coated onboth sides of the board, as well as the above-described embodiment inwhich the board used has been clay-coated on only one side.

It will be understood that other changes may be made in the inventionwithout departing from the spirit thereof, and it is accordingly notintended that the disclosed details of the invention shall be construedas limitations of the scope thereof.

We claim:

1. A process for treating a starch-sized paperboard which comprises,first applying to the board an aqueous solution of borax, then applyingto the borax solution coated board While still wet with the boraxsolution, an

4 aqueous solution of polyvinyl alcohol, the amounts of borax andpolyvinyl alcohol being effective to form a gelled reaction product,followed by the steps of applying a coating of hot wax to the saidgelled reaction product on the thus treated paperboard, and setting thewax by chilling it with cold water.

2. A process in accordance with claim 1 wherein the amount of boraxapplied to the paperboard is from about 0.15 to about 0.25 pound of10-mol borax per thousand square feet of paperboard.

3. A process in accordance with claim 1 wherein the solution of borax isused in a concentration ranging from about 05-20% and the concentrationof the aqueous solution of polyvinyl alcohol is from about 0.5% to about20%.

4. A process in accordance with claim 1 wherein one surface of the saidboard has been clay-coated and wherein the polyvinyl alcohol-borax waxtreatment is applied to the opposite side of said board.

5. A process in accordance with claim 1 wherein one surface of the saidboard has been clay-coated and wherein the polyvinyl alcohol-borax waxtreatment is applied to both sides of said board.

6. A process in accordance with claim 1 wherein the borax solution isapplied by calendering.

References Cited UNITED STATES PATENTS 2,439,108 4/1948 Staehle117-62.). 2,725,306 11/1955 Fisher et al. 117-92 X 2,927,865 3/1960Lewis et al 117-92 X 2,949,382 8/1960 Dickerman et al. 117-84 X3,112,214 11/1963 Roberts et al. 117-60 3,251,709 5/1966 Bonzagni.

3,285,771 11/ 1966 Dabroski.

WILLIAM D. MARTIN, Primary Examiner R. HUSACK, Assistant Examiner US.Cl. XLR.

